A sprain is a state in which injury occurs to a ligament or an area surrounding a joint when abnormal movement which exceeds the permissible range is applied to a movable portion of a joint. In the case of ankle sprains in particular, the outer side of the sole (the side of the small, or pinky toe) becomes immobilized on the ground, and the ankle is pushed to the outside, causing a forceful inward twisting of the bottom of the foot. Such inversion sprains are reported to account for 80-90% of ankle sprains.
When the ankle is in an inverted state, the part that is most readily injured is the anterior talofibular ligament which connects the talus and the fibula at the front of the lateral malleolus. Moreover, when force continues to be exerted, injury can occur to the calcaneofibular ligament or the posterior talofibular ligament at the outer side of the back of the ankle. Accordingly, if an inversion sprain occurs, the ligaments extending between the fibula, the talus, and the calcaneus are injured, resulting in swelling and pain accompanied by internal bleeding in the lateral malleolus, and there are many cases in which the weight of the body cannot be supported immediately after injury.
Accordingly, socks and supporters are conventionally used to prevent sprains. For example, Patent Reference 1 discloses a sprain-preventative sock which limits wobbling and twisting of the ankle beyond what is necessary, by interknitting yarns which contain elastic fibers having strong support, the yarns being disposed in a ring shape at the lower portion of the calf, at the upper part of the ankle, at the base of the instep, and at the base of the toes. An inelastic band is also provided which links the area from the inside and the outside of the calf to the tip of the foot, so as to pass through these ring shape portions longitudinally.    Patent Reference 1: Japanese Laid-Open Patent Application No. 2001-355101
Patent Reference 2 discloses a sock which prevents inversion sprains by sewing a pad which inclines upward from the side of the arch to the outer side of the foot into the inner surface of the sole portion of a sock.    Patent Reference 2: Japanese Laid-Open Utility Model Application No. H5-37218
Patent Reference 3 discloses a tool (brace) for preventing inversion sprains of the ankle, comprising an inflexible belt member wound around the sole to the instep and provided with a first hook-and loop fastener at a lower end, a fastener engaging part disposed at an upper end, a flexible abutting member whose front surface abuts a fastener engaging surface and whose back surface abuts a surface near the ankle when mounted, and a second hook-and loop fastener attached to the abutting member.    Patent Reference 3: Japanese Laid-Open Patent Application No. 2001-238992
However, in the sock of Patent Reference 1, the inflexible band from the inside and outside of the calf to the tip of the foot is designed only to be able to limit movement of the foot as much as possible in a straight line; the sock is not designed to produce a force to resist inward twisting of the foot. Thus, this sock does not provide any means for preventing inversion sprains.
Since the sock of Patent Reference 2 is provided with a pad on the inner surface of the sole portion that inclines upward from the side of the arch to the outer side, the foot is constantly constrained at an angle like a pigeon-toe, so that when a person walks for long periods of time, the weight of the body is always on the outer edges of the feet, thus resulting in the problem that pain and fatigue readily occur. Additionally, because the weight of the body is on the outer edges of the feet, the foot is readily susceptible to eversion, thus increasing the risk of eversion sprains.
In the tool for preventing inversion sprains disclosed Patent Reference 3, work is required to attach and detach the tool to the foot using the belt and the fasteners, and a problem exists in that it is uncomfortable to wear a sock on top of this tool. For elderly persons in particular, it is not easy for them to attach and detach the tool to their ankles, and additional problems exist in that the feet easily become fatigued, and there is a greater risk of stumbling and falling, since the flexible bending movement of the ankle is inhibited in a number of normal walking movements such as: “touching the heel to the ground”→“shifting the body weight forward when the sole of the foot touches the ground”→and “kicking off from the surface of the ground from the tips of the toes.”